1. Field of the Invention
The present invention relates to a device and a method for detecting optical signals and in particular to a device and a method for converting an optical signal into an electrical signal and for reading out the converted electrical signal.
Optical signals are detected e.g. by means of integrated image sensors which are fabricated in MOS technology. A number of image sensor elements are here integrated with additional electronics on an integrated circuit, an IC. The number and the arrangement of the image elements can be varied. For example, one-dimensional arrays, two-dimensional arrays and individual sensors are possible forms.
Various image sensor elements can be realized by means of CMOS technology. Examples are photodiodes, parasitic bipolar transistors, photogates and photosensitive MOSFETs.
To make the image information available to other components of a full system outside the integrated circuit, the output signal of the image sensor element, or the output signals of the plurality of image sensor elements, must first be read out and then be prepared in a suitable way if necessary. Such preparation may e.g. entail amplification, impedance conversion or a characteristic compression in connection with the output signal of the image sensor element.
In this context xe2x80x9creading outxe2x80x9d means that the output signal of the image sensor element is made available to a circuit in the periphery of the image sensor element or of the image sensor array. In such a readout circuit a larger chip area is usually available on the integrated circuit than within the image sensor array. The electrical signal generated by the image sensor element is accessed by the readout circuit via a connection line for the output signal and, possibly, one or more control signals to the image sensor element. It is also known that if all the image sensor elements are not read out simultaneously, each image sensor element has a selection switch to enable all the image sensor elements of an array to be read out.
2. Description of Prior Art
According to the prior art the signals generated by the integrated image sensor elements are accessed using suitable readout circuits, the signals being amplified in the readout circuits. The output signal of image sensor elements is always overlaid with a noise component. Additional noise components accrue due to readout circuit access. It is desirable that these additional noise components be kept as small as possible so that the additional noise component forms only a small part of the total noise component. The reason for this is that if the additional noise component becomes dominant as regards the total noise component, this reduces the quality of the illumination information provided by the integrated image sensor. This is particularly true for small signals, i.e. for an illumination with low brightness.
The readout circuits of image sensor elements and additional subsequent circuits, such as drivers, always contribute to the total noise component. This noise component of the readout circuit or of the additional circuits can only be kept low by investing more effort in the design of the integrated circuit. However, particularly in the case of two-dimensional arrays of image sensor elements, the chip area available on the integrated circuit is generally limited, so that the noise component of the readout circuit relative to the total noise component cannot be kept arbitrarily low.
It is established practice to provide automatic amplification changeover for the read out signal outside an integrated circuit so as to achieve enhanced dynamics or resolution of an analog signal, which is then converted by an A/D converter. This automatic amplification changeover does not, however, contribute to an increase in the signal-noise ratio since all the noise components generated by the readout circuit are also affected by the automatic amplification changeover.
EP 0 135 035 A describes a method for improving the quality of a television picture, wherein an improvement in the image quality is to be achieved through increased amplification of dark parts of the image. The television picture is then displayed on a screen according to the amplification, i.e. with a stronger amplification for dark parts of the image and a smaller amplification for light parts of the image.
The Patent Abstracts of Japan, Vol. 30, No. 268 (E-775), Jun. 20, 1989, disclose an amplifier with adjustable amplification, wherein the amplification is adjusted in accordance with the level of the input signal. The Patent Abstracts of Japan, Vol. 016, No. 465 (E1270), Sep. 28, 1992, disclose an analog/digital converter where an adjustable amplifier is used to obtain a fixed resolution irrespective of an input signal amplitude.
It is the object of the present invention to provide a device and a method for detecting optical signals such that the output signals display an increased signal-noise ratio.
In accordance with a first aspect of the present invention, this object is achieved by an optical detector comprising:
at least one image sensor element, which generates an image sensor element output signal according to the illumination incident on the element;
a readout amplifier switchable between a plurality of amplification stages for the one or each image sensor element for amplifying the respective image sensor element output signal;
an amplification switch for comparing the image sensor element output signal with at least one given threshold value before the signal is amplified by the readout amplifier and for switching between the amplification stages of the readout amplifier depending on the result of the comparison; and
a first output signal indicating the amplified image sensor element output signal and a second output signal indicating the amplification stage to which the amplifier is switched.
In accordance with a second aspect of the present invention this object is achieved by a method for detecting an optical signal, comprising the steps of:
converting the optical signal into an electrical signal by means of an image sensor element;
comparing the electrical signal with at least one given threshold value;
switching the amplification of a readout amplifier between a plurality of amplification stages depending on the result of the comparison;
amplifying the electrical signal according to the amplification stage to which the amplifier is switched to generate an amplified electrical signal; and
outputting a first output signal indicating the amplified electrical signal and a second output signal indicating the amplification stage to which the amplifier is switched.
The present invention is based on performing an automatic amplification changeover during the readout process, i.e. when directly accessing the image sensor element. In this way the influences of the noise components of the readout circuit and of all other circuits, such as drivers, which perform additional analog processing of the output signal, can be reduced through the automatic amplification changeover when accessing the image sensor element directly.
If the output signal of the image sensor element is amplified more for small signals, i.e. for small brightnesses of the illumination, than for larger signals, before it reaches the readout circuit, the ratio of signal component to total noise component at the output of the readout circuit increases. Although the noise component of the output signal from the image sensor element is amplified, just like the signal component of the same, the ratio of the noise component of the readout circuit to the noise component of the image sensor element nevertheless decreases, since the noise component of the readout circuit is not amplified. This results in an improvement in the quality of the illumination information at the output of the readout circuit, i.e. the signal-noise ratio of the output signal of the readout circuit is improved.
The present invention can be used for all known image sensor elements, e.g. those fabricated in CMOS technology, such as photodiodes, parasitic bipolar transistors, photogates and photosensitive MOSFETs. Furthermore, the present invention can be used for arbitrary variations in the number and arrangement of the image elements, e.g. one-dimensional arrays, two-dimensional arrays or individual sensors. Access to the individual image sensor elements can be realized in known ways, e.g. by means of selection switches.